Add support for FreeBSD CURRENT (aka freebsd13)

Using the last FreeBSD-CURRENT (development snapshot) the libc build, but tests fail.
This patch detects and supports FreeBSD CURRENT as freebsd13, and reworks the conditional compilation to use the `freebsd11` attribute instead of `not(freebsd12)`
For now, freebsd13 is reusing all freebsd12 definitions, except for `ELAST`
While here, add a new `errno`introduced in freebsd13

Move musl-compatible constants to linux/mod.rs

This is a continuation of #1452. I think CI is ready to run fully and the PR is ready to be merged if the pipelines pass based on the CI I've done using musl libc in my own projects.

Support Vxworks

Define the C interface to VxWorks

Define the C interface to vxWorks for x86, x86_64, ARM, ARM 64-bit, PowerPC, PowerPC SPE and PowerPC 64-bit.

Upgrade FreeBSD's CI image to release 11.3

skip roundtrip on few structs on OpenBSD

skip roundtrip on some structs on OpenBSD. it is mostly the same than other targets: dirent, utsname, utmp.

define KERN_TIMEOUT_STATS and incr KERN_MAXID on OpenBSD

`KERN_TIMEOUT_STATS` has been defined in [sys/sysctl.h (1.194)](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/sys/sysctl.h?rev=1.194&content-type=text/x-cvsweb-markup)

Add IP_RECVERR (unix/linux_like/mod.rs)

Setup Azure Pipelines

cc @alexcrichton this needs enabling Azure Pipelines for this repo

The 11.3-release GCE image isn't working due to a bug in the included
py36-google-compute-engine package, but that bug is fixed in the latest
stable/11 images.

Currently, libc supports and detects freebsd11 and freebsd13
Unknown versions, like freebsd13, is treated as freebsd11.
This patch solve the issues, detecting freebsd13 and treating it like
freebsd12.
Inverting the logic not(freebsd12) -> freebsd11 where possible

Add support for hexagon-unknown-linux-musl